In numerous tests, particularly in diagnostic tests, certain target substances in a biological sample, in particular in a tissue sample or a body fluid, are to be detected. The target substances are, for example, certain cells such as viral or bacterial pathogens, or specific proteins or nucleic acids of a cell type, tissue type or organism.
For routine clinical practice, systems are desirable in which all the processing and analysis steps are integrated on what is called a cartridge. The word cartridge designates new kinds of biochips with which microbiological tests in particular can be carried out.
Cartridges generally include a microfluidic system of cavities and channels which are necessary, for example, for breakdown of the sample, for cleaning the target molecules, and possibly for amplification and detection (on a microarray). Biochips of this kind can be miniaturized to a check card format and are also referred to by the expression “lab-on-a-chip”. In the text below, the terms “biochip” and “cartridge” are used alongside one another and each designate a “lab-on-a-chip” of this kind.
It would be desirable to be able to use such a cartridge for different sample materials. However, this is not yet possible at present, since different procedures for sample breakdown are needed for different sample materials (for example blood, sputum, biopsies, etc., in the case of samples from humans). Depending on the aim of the test, different sample-processing steps are also needed:
To be able to detect certain intracellular or membrane-bound proteins or nucleic acids in blood or plasma, cell breakup (lysis) is required in the first instance, in order to free the desired target molecules and bring them into solution. For this purpose, only small amounts of blood are generally needed, for example if the aim is to detect human genes or ubiquitous proteins. By contrast, a more complicated type of sample-processing is necessary if the sample material is more compact, e.g. in the case of tissue samples from biopsies, if the cells are robust to chemical agents (e.g. Mycobacterium tuberculosis) of if they are present only in low concentration in the sample (e.g. HIV viruses or Staphylococcus aureus pathogens in urine). In samples of this kind, it is necessary to employ more aggressive chemical lysis reagents, higher temperatures, freeze/thaw cycles and, in some cases, mechanical methods in order to efficiently break up the cells and bring the target molecules into solution. For detection of low concentrations of viral and bacterial pathogens, large quantities of body fluid (>1 ml) have to be processed in order to obtain sufficient material for the subsequent analysis steps. In some cases, the concentration of the pathogens in the tissue sample would have to be increased. For this purpose, ultracentrifugation is used for example, or binding to matrices or resins (chromatographic methods).
Complicated sample-processing steps of this kind can presently only be carried out manually or semi-manually, although this is undesirable in routine clinical practice, on grounds of cost, and for reasons of reproducibility and risk of infection. Alternatively, different cartridges would have to be developed for different sample materials, or an extremely complex and large “universal cartridge” integrating all possible methods would have to be made available.
To avoid the problem of increasing the concentration of viral and bacterial pathogens, it would also be possible to process larger quantities of sample material, but this would again have the consequence of the size of the cartridge increasing and of the assay no longer being able to be carried out at check card size (for example, as in the EDD system from directif).
A particular problem arises when the same test has to be applied as standard to different sample materials. For example, a human genetic test can be carried out both with blood samples and with smear material. However, this requires a different input of the sample and a different processing of the sample. In the fully integrated systems known today, different cartridges would have to be produced for the same test. This would mean disadvantages in terms of production costs and of the work involved in the corresponding approval procedures.